Insight into the cooperation of P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) at the blood-brain barrier: a case study examining sorafenib efflux clearance.
ABSTRACT The ATP-binding cassette transporters P-glycoprotein and breast cancer resistance protein have been shown to be critical determinants limiting drug transport across the BBB into the brain. Several therapeutic agents have been shown to be substrates for these two transporters, and as a result they have limited distribution to the brain. Recently, it has been shown that these two drug transporters cooperate at the BBB and brain penetration of dual substrates increases significantly only when both are absent, e.g., in the Mdr1a/1b(-/-)Bcrp1(-/-) mice. The present study uses the brain penetration of sorafenib to investigate these findings and attempts to explain the mechanistic basis of this cooperation with a simple theory based on affinity and capacity dependent carrier-mediated transport. The brain efflux index method, combined with the organotypic brain slices, was used to determine the net contribution of P-gp and BCRP to the total clearance of sorafenib out of the brain and show that its efflux at the BBB is mediated primarily by BCRP. Sorafenib clearance out of the brain decreased 2-fold in the Bcrp1(-/-) mice and 2.5-fold in the Mdr1a/1b(-/-)Bcrp1(-/-) mice. Clearance out of brain when P-gp was absent did not change significantly compared to wild-type. We also investigated the expression of P-gp and BCRP in the genetic knockout animals and saw no differences in either P-gp or BCRP in the transporter deficient mice compared to the wild-type mice. In conclusion, this study explains the cooperation of P-gp and BCRP by analysis of the efflux clearance of sorafenib and correlating it to the "mechanisms" that determine the clearance, i.e., affinity and capacity.
Article: Expression, up-regulation, and transport activity of the multidrug-resistance protein Abcg2 at the mouse blood-brain barrier.[show abstract] [hide abstract]
ABSTRACT: The breast cancer resistance protein (BCRP/ABCG2) is, like P-glycoprotein (P-gp), a member of the ABC family of drug transporters. These proteins actively transport various anticancer drugs from cells, causing multidrug resistance. The physiological expression of P-gp/ABCB1 at the blood-brain barrier (BBB) effectively restricts the brain uptake of many antitumor drugs by mediating their active efflux from the brain to the blood vessel lumen. However, little is known about the function of Abcg2 at the BBB in vivo. We used in situ brain perfusion to measure the uptake of two known Abcg2 substrates, prazosin and mitoxantrone, and the nonsubstrate vinblastine by the brains of wild-type and P-gp-deficient mutant mdr1a(-/-) mice with or without the P-gp/Abcg2 inhibitor GF120918 or the P-gp inhibitor PSC833. P-gp had no effect on the brain transport of prazosin and mitoxantrone at the mouse BBB, but wild-type and P-gp-deficient mouse brains perfused with GF120918 or a high concentration of prazosin showed carrier-mediated effluxes of prazosin and mitoxantrone from the brain that did not involve P-gp. In contrast, the brain uptake of vinblastine was restricted only by P-gp and not by Abcg2 at the BBB. The amounts of abcg2 mRNA in cortex homogenates and capillary-enriched fractions of wild-type and mdr1a(-/-) mouse brains were measured by real-time quantitative reverse transcription-PCR. There was approximately 700-times more abcg2 mRNA in brain microvessels than in the cortex of the wild-type mice, confirming that Abcg2 plays an important role at the BBB. There was also approximately 3 times more abcg2 mRNA in the microvessels from P-gp-deficient mutant mouse brains than in the microvessels of wild-type mouse brains. These findings confirm that Abcg2 is a physiological transporter at the BBB that restricts the permeability of the brain to its substrates in vivo. Lastly, the defective P-gp in the mutant mdr1a(-/-) mice was associated with increased abcg2 mRNA at the BBB and a greater export of prazosin and mitoxantrone from the brain, as measured in the P-gp-deficient mice versus the wild-type mice.Cancer Research 06/2004; 64(9):3296-301. · 7.86 Impact Factor
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ABSTRACT: The ATP-driven efflux transporter, breast cancer resistance protein (BCRP), handles many therapeutic drugs, including chemotherapeutics, limiting their ability to cross the blood-brain barrier. This study provides new insight into rapid, nongenomic regulation of BCRP transport activity at the blood-brain barrier. Using isolated brain capillaries from rats and mice as an ex vivo blood-brain barrier model, we show that BCRP protein is highly expressed in brain capillary membranes and functionally active in intact capillaries. We show that nanomolar concentrations of 17-β-estradiol (E2) rapidly reduced BCRP transport activity in the brain capillaries. This E2-mediated effect occurred within minutes and did not involve transcription, translation, or proteasomal degradation, indicating a nongenomic mechanism. Removing E2 after 1 h fully reversed the loss of BCRP activity. Experiments using agonists and antagonists for estrogen receptor (ER)α and ERβ and brain capillaries from ERα and ERβ knockout mice demonstrated that E2 could signal through either receptor to reduce BCRP transport function. We speculate that this nongenomic E2-signaling pathway could potentially be used for targeting BCRP at the blood-brain barrier, in brain tumors, and in brain tumor stem cells to improve chemotherapy of the central nervous system.Journal of cerebral blood flow and metabolism: official journal of the International Society of Cerebral Blood Flow and Metabolism 03/2010; 30(10):1742-55. · 5.46 Impact Factor
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ABSTRACT: Transporters govern drug movement into and out of tissues, thereby playing an important role in drug disposition in plasma and to the site of action. The molecular cloning of such transporters has clarified the importance of members of the solute carrier family, such as OATP/SLCO, OCT/SLC22, OAT/SLC22, and MATE/SLC47, and the ATP-binding cassette transporters, such as P-glycoprotein/ABCB1, MRPs/ABCC, and BCRP/ABCG2. Elucidation of molecular characteristics of transporters has allowed the identification of transporters as mechanisms for drug-drug interactions, and of interindividual differences in drug dispositions and responses. Cumulative studies have highlighted the cooperative roles of uptake transporters and metabolic enzymes/efflux transporters. In this way, the concept of a rate-limiting process in hepatic/renal elimination across epithelial cells has developed. This review illustrates the concept of the rate-limiting step in the hepatic elimination mediated by transporters, and describes the prediction of the in vivo pharmacokinetics of drugs whose disposition is determined by transporters, based on in vitro experiments using pravastatin as an example. This review also illustrates the transporters regulating the peripheral drug concentrations.Drug Metabolism and Pharmacokinetics 02/2009; 24(1):37-52. · 2.32 Impact Factor